1. Field of the Invention
The present invention relates to an image reading apparatus such as a document reader, a scanner, a copy machine, a facsimile machine, etc., and particularly to an image reading apparatus having a sheet guide.
2. Description of the Related Art
In the conventional art, there is an image reading apparatus capable of scanning both book-type documents and sheet-type documents. Book-type documents are bound, while sheet-type documents are made of loose-leaf pages. For sheet-type documents, the conventional image reading apparatus employs an automatic document feeder (ADF) to successively feed the sheets to a scanning position where a scanner (or image sensor) reads respective images from the sheets. Because the pages in the book-type document are bound, it is not possible to use the ADF, so in order to scan the pages, the operator opens the book-type document, places the open page(s) on a contact glass, and an image sensor moves beneath the contact glass in order to read the image(s) on the open page(s). Thus, the way in which images are scanned depends on the type of original document.
When the image reading apparatus scans the sheet-type document, the sheets are stacked on a document table and each sheet is successively fed into the ADF and conveyed to a fixed scanning position where the sheet is scanned by the image sensor, which is held in place while the sheet moves across the scanning point, and then the scanned sheet is conveyed to an external discharge tray.
For use with the book-type document, the conventional image reading apparatus includes a pressing board that is a hinged cover and is used to press the book document against the contact glass so as to flatten the subject page(s) against the contact glass. Accordingly, the contact glass must be at least as large as the pages of the book-type document so that the images may be completed scanned. In contrast, when scanning individual sheets of the sheet-type document, a relatively small contact glass is required, because the image sensor remains stationary and thus the contact glass need not be larger than that required to support the stationary image sensor.
As recognized by the present inventor, a difficulty arises in integrating the ADF and contact glass of uniform thickness in a single device that performs both a book-scanning mode of operation and a sheet-scanning mode of operation. In the conventional image reading apparatus, a document table that holds the stack of sheets to be scanned is located on top of the pressing board, adjacent to the ADF. Accordingly, the stack of sheets is elevated with respect to the portion of the contact glass that is used for the sheet-scanning mode of operation. This disparity in height requires that the ADF transport each sheet in a nearly vertical direction (see the curved arrow in FIG. 1) when moving the sheets to the scanning position. As will be discussed, moving the sheet toward the contact glass at such a steep angle gives rise to several problems, including susceptibility to paper jams and image reading synchronization problems. As seen in FIG. 1, each sheet is grasped at a nip portion between a resist roller 1-1 and press roller 1-2, where the movement of the resist roller 1-1 and press roller 1-2 urge the sheet toward the contact glass 1-3 in the direction shown by the arrow.
As presently recognized by the inventor, the steep inclination angle increases the likelihood of the sheet buckling (or crinkling) when a leading edge of the sheet hits the contact glass. In order to lower the angle incidence, and thus avoid the paper jam and synchronization problems, two candidate solutions include lowering the press roller 1-2, and lifting the resist roller 1-1 to a higher position than that shown in FIG. 1.
In the former option, the press roller 1-2 can only be lowered by a slight amount, otherwise the press roller 1-2 will come in contact with the contact glass 1-3. If the press roller 1-2 contacts the contact glass 1-3, the motor that drives the press roller 1-2 will become overloaded and unduly strained. Thus, a clearance is maintained between the press roller 1-2 and contact glass 1-3. In order to further lower the press roller 1-2, the contact glass may be thinned so as to accommodate the press roller 1-2 at a lower position. However, creating the thinned portion increases manufacturing cost, lowers the structural integrity of the contact glass, and adversely affects the uniformity of scanning conditions between the sheet-scanning mode of operation and the book-scanning mode of operation.
Regarding the latter option, the space `A` (see FIG. 2) formed by the resist roller 1-1, press roller 1-2 and contact glass 1-3 is expanded in FIG. 2 to show what happens when the resist roller 1-1 is lifted. When lifted, the space A becomes large and thus, provides a sizable "unguided" area where the sheet fed by the ADF is free to bend and possibly crinkle when the sheet contacts the contact glass 1-3. Moreover, failing to guide the sheet in the space A increases the likelihood that the tip of the sheet will crinkle, and thus jam the ADF. Furthermore, when this crinkling problem occurs, the sheet does not reach the scanning point at a predetermined time period when the scanning operation is initiated. Namely, the image reading apparatus controls the timing of a document reading operation based on a trigger signal sent by a sensor located upstream from the nip between the resist roller 1-1 and press roller 1-2. So, when the tip of the document crinkles, but does not jam the ADF, a timing problem occurs in that the synchronization of the arrival of the sheet to the scanning point and the beginning of a scanning cycle is adversely effected.
Accordingly, the present inventor identified the desirability of providing an upper and a lower sheet guide along a document path in the space `A` to solve this problem. However, it is difficult to provide a lower sheet guide in this region because the contact glass 1-3 extends from the scanning point to beneath the press roller 1-2. Nonetheless, in the conventional device, even if a sheet guide could be included in this area, it would nonetheless be difficult to decrease the angle of incidence (measured between the contact glass and underside of the sheet that faces the press roller 1-2) by changing the position of the resist roller 1-1 or press roller 1-2 without creating other problems, such as burning-out the drive motor for the press roller 1-2 or "crinkling" the respective sheets, which causes jam problems and scanning synchronization problems.